Add a .gitignore for build trees and local scratch.

The tree has no .gitignore at all, so build/, .cache/clangd, __pycache__,
the CTS/vkoverhead/vkmark checkouts and assorted test media had all found
their way into the index.

.gitignore

@@ -0,0 +1,27 @@
+# Build trees
+/build
+/build32
+
+# Editor / tooling state
+/.cache
+/.vscode
+/.directory
+/meson_build_info
+__pycache__/
+*.pyc
+
+# Test suites and benchmarks checked out inside the tree
+/VK-GL-CTS
+/vkoverhead
+/vkmark-master
+
+# Local experiments and scratch data
+/HDD
+/OLD
+/opencl_tests
+/ocl_vendors
+/.icd-vendors
+*.h264
+*.mp4
+/decoded_frame.png
+/test_elk_stateless.patch